Automatic machining centers are previously known which employ a bed which is movable in a horizontal plane in two dimensions. It is also previously known to employ an automatic pallet changer employing a receiver positioned on the bed and having a draw plate for up and down motion of a pallet resting on the draw plate. Such a receiver is disclosed in U.S. Pat. No. 5,492,214. Parts being machined in the automatic machining center are fixed to the pallet by an operator.
Such machining centers have a vertically sliding door for closing off an entrance into the machining center enclosure. At the beginning or end of the machining, the pallet is either moved into or out of the machining enclosure by the automatic pallet changer. When the pallet is outside the enclosure, then it is accessible for a worker to either remove or mount the workpiece being machined.
It has been previously known in U.S. Pat. No. 5,3436,051 to provide a manual pallet change system.
It has been previously disclosed in U.S. application Ser. No. 575,138, entitled "Automatic End Loading Pallet System filed Dec. 19, 1995 recently issued into U.S. Pat. No. 5,813,514, and incorporated by reference herein, to automatically move first and second pallets into or out of the machining enclosure of the machining center. There, first and second pallets are employed so that the worker can be working on one of the pallets while it is outside the machining center while the other pallet is inside the machining enclosure. The pallets can then be automatically exchanged.
Finally, in another type of known automatic pallet system, an automatic pallet changer is provided where first and second tracks are provided side-by-side, with each track having a respective pallet associated with it. A bicycle chain-like horizontal loop is provided between the two tracks. A single dog is provided on the bicycle chain for ultimately engaging with the two pallets. Thus, one pallet is pulled out of the machine center enclosure and thereafter the second pallet is brought into the enclosure as the chain continues rotation.
Disadvantages of the previous systems, and particularly the system disclosed in the '851 patent, are: 1) cost of manufacture is high; 2) the design presents some reliability drawbacks; and 3) the actual pallet change time is excessive.